In the development of more and more improved steel grades, the interest has been concentrated upon the non-metallic inclusions which always exist in steel and which frequently may influence the workability of the steel in a most drastic way and also have a great impact upon its mechanical properties. As well oxidic as sulphidic inclusions may have effect byt in view of the low oxygen content in well killed steels it is natural that the greatest efforts have been concentrated upon the sulphidic inclusions. One has therefore aimed at reducing as well the total content of sulphur in the melt and hence the amount of sulphides, as transforming the remaining sulphides to make them as harmless as possible. To that end one has tried to affect the morphology so that long "worm-like" sulphidic inclusions be transformed to adopt a more spheroidal shape which with reference to the workability of the steel through rolling, forging etc. as well as to its mechanical properties in different directions are more more harmless than the more elongated inclusions.
A particular problem-fields is those low-carbon- and low-silicon aluminium-killed steels which in very great quantities are used in the form of cold-rolled sheet because of the excellent mechanical deep-drawing properties of these steels. A typical range of application is deep-drawing-sheets for motor-car bodies. The fact that these steels may be massproduced at low costs. One way to reduce the production costs is to utilize continuous casting technic. Continuous casting of aluminum killed, low-carbon- and low-silicon steels in practise, however is difficult or impossible to perform due to a number of cooperating factores. Thus the flowility is low, at the same time as large aggregates of aluminium oxides, Al.sub.2 O.sub.3, frequently block the continuous casting nozzles. Therefore one has mainly been thrown upon conventional ingot casting for the production of aluminium killed deep-drawing steels.
However, continuous casting has become the quite dominating method for the production of most other commercial steel grades. For desulphurization purposes and for composition adjustments it has become increasingly frequent to treat the steel melts injection-metallurgically in a ladle. Most of these injection-metallurgical methods have in common that a controlled quantity of a calcium compound, e.g. lime, calcium-carbide and/or calcium-silicon, is injected into the deoxidized steel melt by means of a neutral carrier gas, possibly together with a fluxing agent, such as fluorspar, CaF.sub.2, e.g. as is described in Stahl u. Eisen (1974), No. 11, P 474-485. The desulphurization result basically depends on the total quantity of calcium supplied to the melt, said calcium combining with sulphur existing in the melt to form calcium sulphide, CaS, which is collected in the slag on the surface of the melt, but is substantially independent of which kind of calcium compound that is used. However it has been considered necessary that the supplied mixture contains a high percentage of calcium that is not combined in oxidic form. Such calcium, as distinguished from calcium in the form of lime, may be freed as elementary calcium and in that form be dissolved into the melt and/or react with the inclusions in the melt. This gives the ground for the possible achievement of the desired result as far as the morphology of the inclusions is concerned. At the same time the flowability of the steel is improved through the dissolving of calcium into the melt. Therefore it is a common practice to supply a mixture of calcium compounds which up to fifty percent, or more, consists of calcium-silicon or calcium-carbide, CaC.sub.2, notwithstanding calcium-silicon is about twenty times more expensive than lime. The treatment in other words is comparatively expensive. In the production of low-carbon steels, however even the theoretical possibility of supplying large quantities of calcium-carbide must be neglected because of the recarburization effect. Particularly in the production of deep-drawing steels it is nor possible to supply calcium-silicon in the same way in view of the fact that only very low silicon contents are tolerated in these steels. Nor can mixtures of lime and such an agent as calcium-cyanamide, CaCN.sub.2, a mixture which has been proposed for the desulphurization of steel, according to known technique be used for the treatment of particularly deep-drawing steels because of the recarburization effect.